1. Field of the Invention
The present invention generally relates to a magnetoresistive (MR) sensor having a MR sensor element and a magnetic ruler in which the ruler has alternating magnetic polarization along its length, the sensor element measures the magnetic polarization of the ruler as the sensor element and the ruler move relative to one another, and the sensor element generates a sensor signal indicative of the measured magnetic polarization as a function of the displacement or angular position between the sensor element and the ruler. More particularly, the present invention relates to a method of correcting a characteristic curve derived from measured magnetic polarization values for a measurement range of displacements or angular positions between the MR sensor element and the magnetic ruler of a MR sensor.
2. Background Art
A torque sensor determines torque exerted by a driver on a steering wheel of a motor vehicle. The torque sensor includes an internal hub and an outer rim. The hub torque-locks to a steering shaft of the motor vehicle. Bending spokes connect the hub and the rim. The bending spokes allow the rim to move within a certain rotational angle range with respect to the hub. The hub has outward-projecting limiting spokes for setting a maximum amount of movement between the rim and the hub. The limiting spokes engage in corresponding recesses of the rim such that the range of movement between the rim and the hub is a function of the clearance between the limiting spokes and the recesses of the rim. Accordingly, the range of movement between the rim and the hub extends from no movement up to the maximum amount of movement set by the limiting spokes. Thus, the rim may move relative to the hub at rotational angles falling within the range of movement.
The rim moves relative to the hub when the driver applies torque to the steering wheel. Consequently, the rim and the hub are “offset” from one another by an amount corresponding to the rotational angle of the rim relative to the hub. The offset between two elements such as the hub and the rim represents the displacement or angular position between the two elements.
A torque sensor typically has one or more magnetoresistive (MR) displacement or angular position sensors for detecting the applied torque. Each sensor includes a MR sensor element and a magnetic ruler. The ruler has an alternating magnetic polarization that repeats multiple times over its length. The sensor element and the ruler are movable relative to one another. The sensor element measures the magnetic polarization of the ruler as the sensor element and the ruler move relative to one another. As such, the measured magnetic polarization is indicative of the offset between the sensor element and the ruler.
The sensor element is associated with one of the hub and the rim and the ruler is associated with the other one of the hub and the rim. For example, the sensor element is associated with the hub and the ruler is associated with the rim such that the ruler moves relative to the sensor element as the rim moves relative to the hub. The sensor element measures the magnetic polarization of the ruler as the rim (ruler) moves relative to the hub (sensor element) and generates a sensor signal indicative of the measured magnetic polarization as a function of the offset (i.e., the displacement or angular position) between the hub (sensor element) and the rim (ruler).
The resolution of a torque sensor should be as high as possible and have a correspondingly high accuracy of measurement such that the torque sensor can function with as small as possible an offset between the hub and the rim. As such, efforts are made to make the period of the alternating magnetic polarization of the ruler of the MR sensor be on the order of the magnitude of the maximum possible offset between the hub and the rim such that it is possible to resolve even small offsets between the hub and rim.
The sensor signal generated by a MR sensor for a measurement range of displacements or angular positions between the sensor element and the ruler is evaluated through the arctangent in order to obtain the phase angle of the trigonometric function described by sine and cosine signals having values within a period directly proportional to the displacement or angular position between the sensor element and the ruler. Thus, the arctangent represents a characteristic curve of the MR sensor within the measurement range. A plot of the characteristic curve has measured phase angle values on the “y” axis and displacement or angular position values on the “x” axis.
The selection of the periodic alternating magnetic polarization of the ruler of the MR sensor is limited because the characteristic curve derived from the MR sensor signal may not have any jump discontinuity points within the measurement range. A jump discontinuity jump represents a measured value at a given point in the measurement range which is undefined and therefore cannot be evaluated. As such, the sensor element and the ruler have to be adjusted relative to one another with high accuracy in order that the characteristic curve does not contain any jump discontinuity points within the measurement range. In the periodicity of the alternating magnetic polarization of the ruler it is also necessary to take into account the tolerances to be observed in an arrangement of the sensor element and the ruler to one another when they are being assembled. Therefore, previously known MR displacement or angular position sensors having MR sensor elements and corresponding magnetic rulers have limited resolving power.